This invention relates to fuel cells, such as proton exchange membrane (PEM) fuel cells, in which the fuel reactant gas outlet and the oxidant reactant gas outlet are both adjacent to the coolant inlet thereby providing a more uniform temperature profile across each cell of the fuel cell stack, a lower maximum temperature in each cell of the fuel cell stack, a higher coolant exit temperature, higher water recovery, and further permitting operation with higher air utilization and lower coolant flow.
It is known that, in PEM fuel cells, the membrane must be wet in order to maintain high process efficiency and to avoid membrane degradation which will result in reactant gas crossover. Each proton formed at the anode will drag molecules of water with it as it passes through the membrane to the cathode side, thereby creating a water-pumping effect commonly known as proton drag. The water thus has to be replenished at the anode side of the membrane, continuously, for efficient operation. On the cathode side, water is produced as a byproduct of the process; this is commonly referred to as product water. The proton drag water and product water have to be removed from the cathode in order to prevent the water from blocking the passage of oxidant reactant gas to the cathode catalyst.
It is also known that the useful life of PEM fuel cell membranes is inversely related to their temperature. One of the byproducts of the process is internally generated heat; if hot spots are present in any cells, the membrane deteriorates at such spots and efficiency and life of the fuel cell stack goes down commensurately. Thus, the internally generated heat must be removed throughout each cell of the fuel cell stack in a manner that limits the temperature of hot spots.
In a typical PEM fuel cell stack, the oxidant reactant gas is air, which is provided in excess of the amount necessary to assure adequate oxygen throughout each cell of the fuel cell stack. Because of variations in the cross sectional area of the air flow field channels in each stack, sufficient air has to be provided to satisfy the needs of the cell with the smallest flow field cross sectional area. Higher air utilizations allow use of air pumps which require less electric power to operate, thereby increasing the overall efficiency of the fuel cell stack. However, extremely high overall air utilization results in loss of efficiency in random areas of the fuel cell stack. Thus, a balance must be struck; typical air utilizations may be on the order of 60% (supplying about 1.7 times more air than is necessary for the required oxygen). The unused air carries with it water vapor which is supplied by the proton drag water and product water at the cathode. If the amount of water removed as vapor or liquid in the air or fuel exhausts is too great, then additional water must be supplied in order to ensure that the membrane remains wet, particularly at the anode side.
Water management in a PEM fuel cell must accommodate the foregoing considerations. A desired relationship is to condense sufficient moisture out of the exiting air and fuel streams so that the amount of moisture which is expelled from the fuel cell stack balances the product water. To achieve this, a recent innovation disclosed in U.S. patent application Ser. No. 09/267,416 filed Mar. 12, 1999, now abandoned provides for the coolant inlet to be adjacent to the air outlet so that there is a minimum temperature differential between the exiting air and the entering coolant, thereby achieving substantial condensation of water within the fuel cell stack.
Objects of the invention, in a PEM fuel cell, include: assuring maximum recovery of product water within the fuel cell stack; achieving high performance with increased air utilization; reducing the maximum temperature in each cell of the stack; providing a more uniform temperature profile across each cell of the stack; achieving a higher coolant exit temperature in order to enhance waste heat rejection while having an adequately low coolant inlet temperature to promote condensation in the exiting oxidant reactant gas flow as well as the exiting fuel reactant gas flow; and reducing the flow of coolant in each cell.
According to the present invention, the pattern of fluid flow fields within each cell of a fuel cell stack are arranged so that the oxidant reactant gas outlet and the fuel reactant gas outlet are both adjacent to the coolant inlet, with the coolant exiting each fuel cell adjacent an edge thereof which is opposite to an edge adjacent to the coolant inlet of each fuel cell, and the fuel reactant gas inlet is displaced from the oxidant reactant gas inlet. In one embodiment, the reactant gas flow channels are xe2x80x9ctwo-passxe2x80x9d, flowing from an inlet manifold through half of each cell to a turnaround manifold, within which the gas is redistributed before flowing through the other half of each cell of the stack to an exit manifold. In that embodiment, the fuel reactant gas flow channels are orthogonal to the oxidant reactant gas flow channels, and the exit of the fuel reactant gas is adjacent to the exit of the oxidant reactant gas. Further in that embodiment, the coolant flows into each cell at a point adjacent to the reactant gas outlets, and then flows through flow channels which have three legs and two turns each, to exit each fuel cell at a point diametrically opposite to the coolant inlet.
This invention is predicated in part on the recognition that when fuel reactant flow fields are not dead ended, significant water can be carried out of the fuel cell in the fuel reactant gas exhaust, and is predicated in part on the concept that a balance maintained between rich and partially depleted oxidant reactant gas, partially depleted and rich fuel reactant gas, and coolant temperature will significantly reduce the highest temperature in each fuel cell and commensurately provide a more even temperature profile across each fuel cell.
The invention has been shown to reduce the maximum temperature of a fuel cell stack by over 15xc2x0 F. (about 18xc2x0 C.), while at the same time reducing the coolant flow by about half and increasing the overall oxidant utilization to near 80%. The invention reduces parasitic power loss by allowing smaller pumps for the oxidant reactant gas and for the coolant. The invention causes substantially the warmest part of the fuel cell to be near the coolant exit, which aids in waste heat rejection by permitting significant reduction in the size of an external heat rejection heat exchanger (such as the conventional radiator of an electric vehicle powered by a fuel cell). The condensation of moisture in the exiting fuel also heats the incoming coolant, raising the temperature of the fuel cell stack in that area somewhat, in turn raising the temperature, and thus the vapor pressure, of the incoming air in an adjacent area of the stack, thereby improving humidification of the incoming air.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.